This invention relates to a process and apparatus for the recovery of low-molecular weight olefins from a gaseous stream containing low concentrations of these hydrocarbons and of the corresponding paraffins.
Low-molecular weight olefins, especially ethylene, constitute important starting materials for the chemical industry and are required in large quantities. A conventional large-scale industrial process for the production of ethylene and propylene comprises the pyrolysis of hydrocarbons, using high cracking temperatures and short residence times. Such processes usually lead to a product gas containing, in a typical cracking operation based on naphtha and gas oil, between 20% and 30% by weight of ethylene. This cracked gas, after cooling and separation of condensed less volatile materials, is fractionated into its individual components in a low temperature gas separation facility.
Economically feasible operation of such a low-temperature separation facility requires that the concentration of the desired low-molecular weight olefins in the feed gas be at a concentration of generally at least on the order of about 20% by weight, varying according to plant investment and operating costs of the particular facility as well as to the market price for the final products.
A lower concentration of the olefins generally results in an economically unattractive low-temperature separation, the lower the concentration, the poorer the economics.
In a number of large-scale industrial processes not having a major objective of obtaining low-molecular weight olefins, there are nevertheless by-product or waste gaseous streams produced containing such a low concentration of olefins that the low temperature recovery thereof is not economically feasible. Such a gas is, for example, the waste gas from a Fischer-Tropsch synthesis oriented primarily to the production of hydrocarbons boiling in the gasoline range from a synthesis gas of hydrogen and carbon oxides. The waste gas produced in such a process contains mostly unreacted synthesis gas along with a minor quantity of light hydrocarbons formed during the synthesis, such as methane, ethane, ethylene, propane, propylene, and C.sub.4 -hydrocarbons. The ethylene or propylene content ranges, for example, at about 2-3 mol-% in a conventional Fischer-Tropsch plant.
Additional information concerning the prior art processes is found in U.S. Pat. No. 2,915,881, discussed in greater detail below.